Dark trace screen



Fatented 6d.

DAB/K TRACE SCREEN Humboldt W. 'Leverenz, Princeton, N. 5., ass'ighor "to Radio Corporation of America, a corporation of Delaware No Drawing. Application -l e cember-20, 1943, Serial N0. 515,050

4 Claims.

My invention relates to cathoderay tubes and more particularly to tubes having'targets scanned by. an electron beam and comprising materials such as an alkali metal halides which change color under electron bombardment and to the method of manufacturing such targets.

This application is a continuation-in-part of my application, Serial No. 451,871, filed July 22, 1942, issued as Patent No. 2,432,908, December 16, 1947.

Crystals of alkali metal halides, notably potassium chloride, are normally transparent or translucent and have the'property of developing in the crystal under eleetron bombardment colors or color centers of a'density dependent upon the instantaneous intensity of the electron beam. Upon cessation of the electron bombardment these colors or color centers gradually fade and, the crystal returns to its original uncolored condition. If a target composed of crystals of alkali metal halides is scanned by an electron beam, the :path of the beam will be shown by a coloredtrace which will persist fora timedependent upon the rate at which the color centers in a bombarded crystal fade out and disappear after the beam moves away from the crystal. Repeated bombardment or very strong bombardment by cathode rays of alkali metal halide screens or targets produces a coloration which is difiicult to eradicate by conventional means such asheat, illumination Within the coloration absorption band or the application of electric fields through the target.

It is an object of my invention to develop color centers and control their dissipation more eflectively in an alkali metal halide crystal screen. ,It is another object to accelerate the rate of return to their original uncolored condition of alkali metal halide crystals in which color centers have appeared as a result of electron bombardment.

A still further object is to provide a more stable and more easily controlled cathode ray target of the alkali metal halide type. I

One type of cathode ray tube utilizing a target made 'in accordance with my invention, wherein the effect of. the electron beam trace on the target n'i'a'y be viewed either by reflected or transmitted light, comprises, as shown in the parent case, my application Serial No. 451,871, a'highly evacuated cylindrical envelope or bulb with flat ends and a neck. or arm enclosing a conventional electron gun and projecting from one side of the bulb so that the gun does not obstruct the view through the ends of the tube. The flat target is it mounted in and extends transversely of the cylindrical bulb in position to be scannedby amodulated beam from the electron gun. Light from 'asubstantially constant light source, andpie'ferably formed into parallel light raysby a lens "systern, m'ay'be pro'jected'through'one endo'f th cylindrical bulb and upon the target, which may if desired *be on or deposited upon the inner wall of the other end of the bulb. The target may 'b'e'viewed through that end of the tub which is opposite the light source and the scanned side of "the target, in which case the light is transmitted through the target, or may be viewed from the "scanned side by observing it through the other end of th tube next 'to the light source, or may be viewed by reflected light from the unscanned side, or optical system-s may be used to project the image of the target upon a viewing screen for observation. In efiect, the colored trace produced by the electron beam is viewed against a background composed of those non-excited portions of the target which are not under electron bombardment and which have a color dependent on the colorof the light source. The colors or color centers developed in alkali metal halide targets by electron bombardment have a relatively narrow spectral absorption characteristic, so that only that portion of the light from a relatively wide spectral range light source which is within the narrow absorption characteristic is absorbed by the color centers in the halide target. The light source should have a spectral emission band falling at least partially, or preferably entirely, within the spectral absorption band of the color centers of the halide target so that the contrast between the light transmission or reflection of the excited areas and that of non-excited areas oithe target may be distinct.

In accordance with my invention I accelerate and thereby control to better advantage the rate atwhich the color centers of the halide crystals fade or are dissipated upon cessation of electron bombardment by introducing a deficiency struc-- ture in the halide crystals. More particularly, 1 synthesize the halide crystals in such a manner and of such a composition that a considerable number of the crystal lattice units, that is, ions are missing from the normally complete crystal structure, which contains controlled crystal fault formations induced by various concentrations of certainingredients. Preferably the major constituent of the halide crystals is an alkali metal halide. All of the alkali metal halides used heretofore consisted of monovalent cations and anions and in accordance with my invention I partially substitute polyvalent cations for some of the monovalent cations of the alkali metal halide in order to induce a deficiency structure in the alkali metal halide crystal lattice. The movement of electrons through the crystal and the concentration of color centers are strongly influenced by vacant and interstitial lattice units. The substituent cation should have a higher valency than that of the ion it replaces. For example, by co-crystallizing a small amount of a halide of a polyvalent metal having a valence greater than unity with the monovalent alkali metal halide, the halogen anion positions are apparently filled throughout the resulting crystal lattice, but for every cation of a bivalent metal, there will be a vacant alkali metal cation position. Consequently, the crystal lattice has vacant alkali metal cation positions which are conducive to more rapid dissipation of the color centers following their formation by the cathode ray beam. The dissipation of the color centers is thereby controlled and may be further accelerated by the application of heat, illumination within the coloration of the absorption band of the halide screen, or by the application of electric fields across the crystal boundaries.

Bivalent metals selected from the metals of group two of the periodic'system, such as magnesium, barium and other alkaline earth metals,

zinc, and cadmium may be utilized for providing the deficiencies in the crystal lattice of the alkali metal halide target. In the preferred form of my invention the bivalent metal is in the form of the same halide as the halide of the alkali metal and is a halide, such as the chloride or K bromide, which co-crystallizes with the corresponding halide of the alkali metal. The mixed halides condensed from the vapor phase will cocrystallize into co-crystals which are stable and will withstand the temperature of about 400 C. to 450 C. in vacuum reached during conventional exhaust of a cathode ray tube.

In a preferred form of my improved alkali metal halide screen I use potassium chloride as the alkali metal halide, plus 29% MgCl26H2O as a source of abivalent metal, magnesium, for inducing a deficiency structure in the potassium chloride crystallattice. Thus, as a specific example, a mixture of 0.367 gram (0.0049 mole) of pure potassium chloride (KCl) and 0.151 gram (0.00074 mole) of pure magnesium chloride (MgClzGHzO) is carefully heated in a platinum container by radiation to minimize decrepitation, to a red heat (500 C.-l000 C.) for a sufiicient time to melt or at least to well sinter the two salts into a compact mass of treated halide to produce the alkali metal halide screen. In this case there is l 0.00074/0.0049=15 mol-percent of Mg (or MgClz).

I have obtained good results With screens composed of potassium chloride plus either cadmium chloride (CdCl222H2O) or zinc chloride (ZnClz). Potassium chloride plus.25% of zinc chloride or plus 7.5% of cadmium chloride gave transparent films which did not opacify readily upon standing in air for 24 hours. Potassium chloride plus 7.5% cadmium chloride gave a quite transparent film which became more translucent upon exposure to air for 24 hours and from which a good dark trace was obtained.

The compact mass of co-crystallized potassium chloride and magnesium chloride in a platinum container is placed in an evacuated vessel, such as the bulb of the tube, and heated by radiant energy or by an insulated resistance heater coil surrounding the container, thereby vaporizing in vacuum the prepared mass of chlorides and condensing the chloride vapors upon the target foundation. The container may be of some refractory material other than platinum if it is coating or layer of alkali metal halide crystals consisting of co-crystallized potassium chloride and magnesium chloride, and having potassium deficiencies in their crystal lattices.

Another means of producing the target comprises co-evaporatin KC] and MgClz separately from individual heaters.

The halide target prepared as above described or in an evacuated vessel other than the tube bulb and then transferred to the tube bulb, maybe treated by subjecting it to an atmosphere con-- taining either water vapor, ammonia or both, followed by sealing and evacuating the'bulb of the tube prior to the operation of the tubepThis treatment decreases the transparency of the tar get to white Iight and renders it more milky in appearance and appears to increase the contrast during use. The co-crystallized salts may also be comminuted and deposited upon the target foundation by settling from a suitable gas or liquid suspension suchas air oramyl-acetate.

Both the kind and the amount of the substif tuted ion or ions may be varied widely with respect to the total amount of the alkali metal halide base, although I prefer to utilize not more than 30 mol-percent of a bivalent substituent elef ment based on the mol-Weight of the base mate rial element for which it is substituted. More particularly a smaller amount than30% is preferable and I have found approximately one-sixth of this amount or 5% of the bivalent metalintroduced into any alkali metal halide, such as potassium chloride, reduces thejtime normally required ,for

' strongly colored traces to lose substantially all their color by. a factor'of approximately 50 at room temperature. The concentration of vacant lattice positions and hence the rate of dissipation of the color centers may be'varied by varyin the concentrations of the substituted bivalent ionsL,

While I have described my invention with particular reference'to the'preparation and use of a target wherein color centers are developedby corpuscular energy, such as a cathode ray beam, it will be appreciated that these color centers may be developed by undulatory energy such as radiae tion with suitable Wavelengths of X ray or by ultra-violet light without departing from. the scope of my invention asset forth in the appended claims.

I claim:

1. An electric discharge device containing a film consisting of a mixture sensitive to the impact of electrons, said mixtureconsisting mainly of potassium chloride and a minor proportion of magnesium chloride.

2. An electric discharge device containing a film consisting of a mixture sensitive to the impact of electrons, said mixture consistin mainly of potassium chloride and a minor proportionof 5. of a member from the group consisting of mag- Number nesium, zinc, cadmium, and barium. '9 4. A cathode ray screen consisting mainly of 2163918 potassium chloride and approximately 5% of 2233786 cadmium chloride. 5 2:303:576 2,330,171 HUMBOLDT W. LEVERENZ. 2,338,233

REFERENCES CITED The following references are of record in the 10 Number Name Date DeBoer Apr. 10, 1934 Schwartz June 27, 1939 Law Mar. 4, 1941 Nelson Dec. 1, 1942 Rosenthal Sept. 21, 1943 Dimmick Jan. 4, 1944 FOREIGN PATENTS Country Date Great Britain Sept. 26, 1938 Certificate of Correction Patent No. 2,451,292

October 12, 1948 HUMBOLDT W. LEVERENZ It is hereby certified that error appears in the printed specificati numbered patent requiring correction as follows:

on of the above 'ne '72, after the word THOMAS F. MURPHY,

Assistant Oomam'ssz'oner of Patents. 

